How to implement the Hashable Protocol in Swift for an Int array (a custom string struct)

I am making a structure that acts like a String, except that it only deals with Unicode UTF-32 scalar values. Thus, it is an array of UInt32. (See this question for more background.)

What I want to do

I want to be able to use my custom ScalarString struct as a key in a dictionary. For example:

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  • var suffixDictionary = [ScalarString: ScalarString]() // Unicode key, rendered glyph value
    
    // populate dictionary
    suffixDictionary[keyScalarString] = valueScalarString
    // ...
    
    // check if dictionary contains Unicode scalar string key
    if let renderedSuffix = suffixDictionary[unicodeScalarString] {
        // do something with value
    }
    

    Problem

    In order to do that, ScalarString needs to implement the Hashable Protocol. I thought I would be able to do something like this:

    struct ScalarString: Hashable {
    
        private var scalarArray: [UInt32] = []
    
        var hashValue : Int {
            get {
                return self.scalarArray.hashValue // error
            }
        }
    }
    
    func ==(left: ScalarString, right: ScalarString) -> Bool {
        return left.hashValue == right.hashValue
    }
    

    but then I discovered that Swift arrays don’t have a hashValue.

    What I read

    The article Strategies for Implementing the Hashable Protocol in Swift had a lot of great ideas, but I didn’t see any that seemed like they would work well in this case. Specifically,

    • Object property (array is does not have hashValue)
    • ID property (not sure how this could be implemented well)
    • Formula (seems like any formula for a string of 32 bit integers would be processor heavy and have lots of integer overflow)
    • ObjectIdentifier (I’m using a struct, not a class)
    • Inheriting from NSObject (I’m using a struct, not a class)

    Here are some other things I read:

    • Implementing Swift’s Hashable Protocol
    • Swift Comparison Protocols
    • Perfect hash function
    • Membership of custom objects in Swift Arrays and Dictionaries
    • How to implement Hashable for your custom class
    • Writing a good Hashable implementation in Swift

    Question

    Swift Strings have a hashValue property, so I know it is possible to do.

    How would I create a hashValue for my custom structure?

    Updates

    Update 1: I would like to do something that does not involve converting to String and then using String‘s hashValue. My whole point for making my own structure was so that I could avoid doing lots of String conversions. String gets it’s hashValue from somewhere. It seems like I could get it using the same method.

    Update 2: I’ve been looking into the implementation of string hash codes algorithms from other contexts. I’m having a little difficulty knowing which is best and expressing them in Swift, though.

    • Java hashCode algorithm
    • C algorithms
    • hash function for string (SO question and answers in C)
    • Hashing tutorial (Virginia Tech Algorithm Visualization Research Group)
    • General Purpose Hash Function Algorithms

    Update 3

    I would prefer not to import any external frameworks unless that is the recommended way to go for these things.

    I submitted a possible solution using the DJB Hash Function.

    4 Solutions Collect From Internet About “How to implement the Hashable Protocol in Swift for an Int array (a custom string struct)”

    This answer has been completely rewritten after submitting my original answer to code review.

    How to implement to Hashable protocol

    The Hashable protocol allows you to use your custom class or struct as a dictionary key. In order to implement this protocol you need to

    1. Implement the Equatable protocol (Hashable inherits from Equatable)
    2. Return a computed hashValue

    These points follow from the axiom given in the documentation:

    x == y implies x.hashValue == y.hashValue

    where x and y are values of some Type.

    Implement the Equatable protocol

    In order to implement the Equatable protocol, you define how your type uses the == (equivalence) operator. In your example, equivalence can be determined like this:

    func ==(left: ScalarString, right: ScalarString) -> Bool {
        return left.scalarArray == right.scalarArray
    }
    

    The == function is global so it goes outside of your class or struct.

    Return a computed hashValue

    Your custom class or struct must also have a computed hashValue variable. A good hash algorithm will provide a wide range of hash values. However, it should be noted that you do not need to guarantee that the hash values are all unique. When two different values have identical hash values, this is called a hash collision. It requires some extra work when there is a collision (which is why a good distribution is desirable), but some collisions are to be expected. As I understand it, the == function does that extra work. (Update: It looks like == may do all the work.)

    There are a number of ways to calculate the hash value. For example, you could do something as simple as returning the number of elements in the array.

    var hashValue: Int {
        return self.scalarArray.count
    } 
    

    This would give a hash collision every time two arrays had the same number of elements but different values. NSArray apparently uses this approach.

    DJB Hash Function

    A common hash function that works with strings is the DJB hash function. This is the one I will be using, but check out some others here.

    A Swift implementation provided by @MartinR follows:

    var hashValue: Int {
        return self.scalarArray.reduce(5381) {
            ($0 << 5) &+ $0 &+ Int($1)
        }
    }
    

    This is an improved version of my original implementation, but let me also include the older expanded form, which may be more readable for people not familiar with reduce. This is equivalent, I believe:

    var hashValue: Int {
    
        // DJB Hash Function
        var hash = 5381
    
        for(var i = 0; i < self.scalarArray.count; i++)
        {
            hash = ((hash << 5) &+ hash) &+ Int(self.scalarArray[i])
        }
    
        return hash
    } 
    

    The &+ operator allows Int to overflow and start over again for long strings.

    Big Picture

    We have looked at the pieces, but let me now show the whole example code as it relates to the Hashable protocol. ScalarString is the custom type from the question. This will be different for different people, of course.

    // Include the Hashable keyword after the class/struct name
    struct ScalarString: Hashable {
    
        private var scalarArray: [UInt32] = []
    
        // required var for the Hashable protocol
        var hashValue: Int {
            // DJB hash function
            return self.scalarArray.reduce(5381) {
                ($0 << 5) &+ $0 &+ Int($1)
            }
        }
    }
    
    // required function for the Equatable protocol, which Hashable inheirits from
    func ==(left: ScalarString, right: ScalarString) -> Bool {
        return left.scalarArray == right.scalarArray
    }
    

    Other helpful reading

    • Which hashing algorithm is best for uniqueness and speed?
    • Overflow Operators
    • Why are 5381 and 33 so important in the djb2 algorithm?
    • How are hash collisions handled?

    Credits

    A big thanks to Martin R over in Code Review. My rewrite is largely based on his answer. If you found this helpful, then please give him an upvote.

    Update

    Swift is open source now so it is possible to see how hashValue is implemented for String from the source code. It appears to be more complex than the answer I have given here, and I have not taken the time to analyze it fully. Feel free to do so yourself.

    One suggestion – since you are modeling a String, would it work to convert your [UInt32] array to a String and use the String‘s hashValue? Like this:

    var hashValue : Int {
        get {
            return String(self.scalarArray.map { UnicodeScalar($0) }).hashValue
        }
    }
    

    That could conveniently allow you to compare your custom struct against Strings as well, though whether or not that is a good idea depends on what you are trying to do…

    Note also that, using this approach, instances of ScalarString would have the same hashValue if their String representations were canonically equivalent, which may or may not be what you desire.

    So I suppose that if you want the hashValue to represent a unique String, my approach would be good. If you want the hashValue to represent a unique sequence of UInt32 values, @Kametrixom’s answer is the way to go…

    Edit (31 May ’17): Please refer to the accepted answer. This answer is pretty much just a demonstration on how to use the CommonCrypto Framework

    Okay, I got ahead and extended all arrays with the Hashable protocol by using the SHA-256 hashing algorithm from the CommonCrypto framework. You have to put

    #import <CommonCrypto/CommonDigest.h>
    

    into your bridging header for this to work. It’s a shame that pointers have to be used though:

    extension Array : Hashable, Equatable {
        public var hashValue : Int {
            var hash = [Int](count: Int(CC_SHA256_DIGEST_LENGTH) / sizeof(Int), repeatedValue: 0)
            withUnsafeBufferPointer { ptr in
                hash.withUnsafeMutableBufferPointer { (inout hPtr: UnsafeMutableBufferPointer<Int>) -> Void in
                    CC_SHA256(UnsafePointer<Void>(ptr.baseAddress), CC_LONG(count * sizeof(Element)), UnsafeMutablePointer<UInt8>(hPtr.baseAddress))
                }
            }
    
            return hash[0]
        }
    }
    

    Edit (31 May ’17): Don’t do this, even though SHA256 has pretty much no hash collisions, it’s the wrong idea to define equality by hash equality

    public func ==<T>(lhs: [T], rhs: [T]) -> Bool {
        return lhs.hashValue == rhs.hashValue
    }
    

    This is as good as it gets with CommonCrypto. It’s ugly, but fast and not manypretty much no hash collisions for sure

    Edit (15 July ’15): I just made some speed tests:

    Randomly filled Int arrays of size n took on average over 1000 runs

    n      -> time
    1000   -> 0.000037 s
    10000  -> 0.000379 s
    100000 -> 0.003402 s
    

    Whereas with the string hashing method:

    n      -> time
    1000   -> 0.001359 s
    10000  -> 0.011036 s
    100000 -> 0.122177 s
    

    So the SHA-256 way is about 33 times faster than the string way. I’m not saying that using a string is a very good solution, but it’s the only one we can compare it to right now

    It is not a very elegant solution but it works nicely:

    "\(scalarArray)".hashValue
    

    or

    scalarArray.description.hashValue
    

    Which just uses the textual representation as a hash source